Gas pipelines are used to transport natural gas from arctic regions to other regions in which the gas is to be used. These pipelines may be refrigerated where they pass through permafrost in order to prevent settlement of the pipeline during spring and summer thaws and other conditions related to thawing permafrost, such as soil erosion, icings and slope instability. It is also economically advantageous to chill high pressure gases in large diameter pipelines so as to increase gas density and thereby lower compression costs and reduce fluid flow pressure losses.
When such refrigerated pipelines traverse unfrozen ground or shallow permafrost where the soil is frost-susceptible, damage to the pipeline may occur due to what is known as "frost heaving". Frost heaving is caused by the migration of water toward a cold pipeline and the freezing of this water beneath the pipeline in sheets or "lenses" of almost pure ice. The resulting accumulation of ice can cause soil movement and corresponding pipe deformation, usually in the form of heaving the soil and pipeline upward above these bodies of ice. In addition, different soils heave at different rates which raises the possibility of differential heaving and uneven stress on the pipeline, which may be an even greater threat to pipeline integrity. For example, a pipeline may encounter a region of unfrozen frost-susceptible ground adjacent to a region of permafrost ground. When the frost-susceptible region of the ground freezes due to cooling by the pipeline, it will heave upward much more rapidly than will the surrounding permafrost region. The resulting stresses due to this differential heaving can cause deformation and, ultimately, rupture of the pipeline.
Bodies of ice tend to form beneath the pipeline primarily because this is usually the coldest region of the temperature field around the pipeline. As more water migrates to this region, it freezes and continues to expand and thicken the bodies of ice, which thereby exert increasing upward pressure tending to uplift the pipeline. As the bodes of ice grow in size beneath the pipeline, they move the soil and therefore the pipeline at a rate which depends on many factors, including the type of soil, the distribution of upward force which in turn depends upon the distribution of the thickening ice bodies, the availability of water and the rate of its migration, and the pressure and shear characteristics of the overburden which depends upon the type and amount of overburden material.
The top of a pipeline may be at least 30 inches below the surface of adjacent soil and diameters of large pipelines may range from about 36 to about 56 inches. Cooling and compressing stations may be located at intervals along such pipelines so that the gas can be maintained at a high pressure, such as about 1,000 to about 2,100 psig, and at temperatures between about 10.degree. F. and about 30.degree. F., more usually from about 15.degree. F. to about 25.degree. F.
Several methods have been proposed in the past for dealing with the problem of frost-heave of chilled gas pipelines. These include replacing frost-susceptible soil around the pipeline with non-frost susceptible soil and physically restraining the pipeline against soil heaving. Another proposed solution has been to heavily insulate the pipeline and to heat the soil beneath the pipeline in order to prevent formation of ice bodies beneath the pipeline.
While the methods of the prior art may provide some measure of relief from the problems of frost-heaving, each has serious disadvantages. Such prior art methods generally require specialized construction techniques, such as providing the pipelines with insulating material or individual heating units at numerous locations along the length of the pipeline. Where heaters are used, careful surveillance and frequent adjustments of heating rates may be required. Furthermore, such specialized methods and apparatus inherently involve relatively high costs that may prove to be prohibitive due to the length of a pipeline system needing frost-heave protection. There also may be hundreds of transitions from frozen to frost-susceptible ground associated with a major natural gas pipeline from an arctic region.